Steering control apparatus

ABSTRACT

A steering ECU includes a plurality of microcomputers to control a motor. A main communication line for communicating a main command value and a subsidiary communication line for communicating a subsidiary command value are connected to each of the microcomputers. The steering ECU includes an abnormality detecting circuit configured to detect a normal state, a semi-normal state detected during the normal state, or an abnormal state detected during the semi-normal state as a communication state of each communication line. When the normal state is detected for the main communication line, the abnormality detecting circuit is configured to set use of the main command value. When the semi-normal state or the abnormal state is detected for the main communication line, the abnormality detecting circuit is configured to set use of the subsidiary command value under a condition that the normal state is detected for the subsidiary communication line.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-254201 filed onDec. 27, 2016 including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a steering control apparatus.

2. Description of the Related Art

There is provided a steering control apparatus including a controlprocessing circuit configured to control driving of a motor that is asource of power for turning steered wheels of a vehicle so that thepower is applied to a steering mechanism (for example, Japanese PatentApplication Publication No. 2007-153001 (JP 2007-153001 A)).

JP 2007-153001 A describes a steering control apparatus including afirst processing unit configured to compute a motor command value, and asecond processing unit configured to control driving of a motor based onthe motor command value computed by the first processing unit. Thoseprocessing units are connected to each other by two communication linesof an active system and a backup system through which the processingunits are communicable with each other. In the two communication linesof the active system and the backup system, the same information isbasically transmitted as information for controlling the driving of themotor except for identification information indicating the active systemor the backup system. Based on the identification information, therespective processing units use information communicated through thecommunication line of the active system when controlling the driving ofthe motor.

In the steering control apparatus described in JP 2007-153001 A, when anabnormality such as interruption of communication is recognized in thecommunication line of the active system, the content of theidentification information is switched, so that the currentcommunication line of the backup system is switched to a communicationline of an active system. Thus, even when the abnormality such asinterruption of communication is recognized in the communication line ofthe active system, disruption of communication is prevented between theprocessing units.

In the steering control apparatus described in JP 2007-153001 A, whenthe abnormality such as interruption of communication is recognized inthe communication line of the active system, the content of theidentification information is switched, so that the currentcommunication line of the backup system is switched to a communicationline of an active system. In this case, however, the abnormality hasalready occurred. That is, even when the communication line of thebackup system is switched to a communication line of an active system,the respective processing units execute the control under the conditionthat the abnormality has occurred. Thus, there is a situation in whichthe power cannot appropriately be applied to the steering mechanism.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a steering controlapparatus capable of reducing the occurrence of a situation in whichpower cannot appropriately be applied to a steering mechanism.

A steering control apparatus according to one aspect of the presentinvention includes:

control processing circuits configured to control driving of a motorthat is a source of power for turning a steered wheel of a vehicle sothat the power is applied to a steering mechanism; and

a plurality of communication lines for communicating control informationnecessary for the control processing circuits to control the driving ofthe motor, the plurality of communication lines being connected to eachof the control processing circuits.

The plurality of communication lines include a main communication lineand a subsidiary communication line. The main communication line is usedfor communicating main control information that is set so as to bemainly used by the control processing circuits as the controlinformation. The subsidiary communication line is used for communicatingsubsidiary control information that is structured so as to be usable bythe control processing circuits as the control information in place ofthe main control information.

The steering control apparatus further includes an abnormality detectingcircuit configured to detect a normal state, a semi-normal statedetected during the normal state, or an abnormal state detected duringthe semi-normal state as a communication state of each of the pluralityof communication lines. The normal state is a state in which the controlinformation is communicable. The semi-normal state is a state in whichan abnormality occurs but the control information is communicable. Theabnormal state is a state in which an abnormality occurs and the controlinformation is not communicable.

When the normal state is detected for the main communication line, theabnormality detecting circuit is configured to set use of the maincontrol information communicated via the main communication line. Whenthe semi-normal state or the abnormal state is detected for the maincommunication line, the abnormality detecting circuit is configured toset use of the subsidiary control information communicated via thesubsidiary communication line under a condition that the normal state isdetected for the subsidiary communication line.

In the configuration described above, the use of the main controlinformation communicated via the main communication line havinginstability in its communication state can be stopped in a stage inwhich the main communication line is in the semi-normal state before theabnormal state is detected. In this case, detection of the normal statefor the subsidiary communication line is set as the condition.Therefore, the control of the driving of the motor can be continued byusing the subsidiary control information in place of the main controlinformation. Thus, it is possible to reduce the occurrence of asituation in which the control information communicated via thecommunication line having instability or abnormality in itscommunication state is used for the control of the driving of the motor.Accordingly, it is possible to reduce the occurrence of the situation inwhich the power cannot appropriately be applied to the steeringmechanism even in the situation in which the communication state of themain communication line has instability.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention willbecome apparent from the following description of example embodimentswith reference to the accompanying drawings, wherein like numerals areused to represent like elements and wherein:

FIG. 1 is a diagram illustrating an overview of an electric powersteering system;

FIG. 2 is a block diagram illustrating the electrical configuration of asteering ECU that implements a steering control apparatus;

FIG. 3 is a block diagram illustrating functions of microcomputers ofthe steering ECU;

FIG. 4 is a block diagram illustrating functions of an input valueprocessing circuit of each of the microcomputers;

FIG. 5 is a diagram illustrating communication states of a communicationline;

FIG. 6 is a diagram illustrating a relationship between a vehicle speedand a limiting threshold;

FIG. 7 is a flowchart illustrating input value processing to be executedby each of the microcomputers;

FIG. 8 is a flowchart illustrating subsidiary information effectivenessdetermining processing in the input value processing;

FIG. 9 is a flowchart illustrating main information effectivenessdetermining processing in the input value processing;

FIG. 10 is a flowchart illustrating information selecting processing inthe input value processing; and

FIG. 11 is a flowchart illustrating input value limiting processing inthe input value processing.

DETAILED DESCRIPTION OF EMBODIMENTS

A steering control apparatus according to one embodiment of the presentinvention is described below. As illustrated in FIG. 1, an electricpower steering system (automatic steering system) 1 is mounted on avehicle A. The electric power steering system 1 has a function ofadvanced driver assistance systems (ADAS), that is, a function ofassisting a driver in his/her driving.

The electric power steering system 1 includes a steering mechanism 2 andan actuator 3. The steering mechanism 2 turns steered wheels 15 based ona user's operation for a steering wheel 10. The actuator 3 applies power(assist force) for turning the steered wheels 15 to the steeringmechanism 2.

The steering mechanism 2 includes the steering wheel 10 and a steeringshaft 11. The steering wheel 10 is operated by the user. The steeringshaft 11 is fixed to the steering wheel 10. The steering shaft 11includes a column shaft 11 a, an intermediate shaft 11 b, and a pinionshaft 11 c. The column shaft 11 a is coupled to the steering wheel 10.The intermediate shaft 11 b is coupled to the lower end of the columnshaft 11 a. The pinion shaft 11 c is coupled to the lower end of theintermediate shaft 11 b. The lower end of the pinion shaft 11 c iscoupled to a rack shaft 12 via a rack and pinion mechanism 13.Rotational motion of the steering shaft 11 is converted intoreciprocating linear motion in an axial direction of the rack shaft 12via the rack and pinion mechanism 13. The reciprocating linear motion istransmitted to the right and left steered wheels 15 via tie rods 14coupled to both ends of the rack shaft 12, thereby changing steeredangles of the steered wheels 15.

The actuator 3 is provided in the middle of the column shaft 11 a fixedto the steering wheel 10. The actuator 3 includes a motor 20 that is asource of the power (assist force) to be applied to the steeringmechanism 2. For example, the motor 20 is a surface permanent magnetsynchronous motor (SPMSM), and is a three-phase brushless motorconfigured to rotate based on driving electric power of three phases (U,V, and W). A rotary shaft 21 of the motor 20 is coupled to the columnshaft 11 a via a speed reducing mechanism 22. The actuator 3 transmits arotational force of the rotary shaft 21 of the motor 20 to the columnshaft 11 a via the speed reducing mechanism 22. The torque (rotationalforce) of the motor 20 that is applied to the column shaft 11 a servesas the power to change the steered angles of the right and left steeredwheels 15.

As illustrated in FIG. 1 and FIG. 2, the motor 20 includes a rotor 23and a stator 24. The rotor 23 rotates about the rotary shaft 21 of themotor 20. The stator 24 is arranged on the outer periphery of the rotor23. Permanent magnets are fixed to the surface of the rotor 23. Thepermanent magnets are arranged side by side in a circumferentialdirection of the rotor 23 so that different poles (N pole and S pole)alternate with each other. When the motor 20 rotates, the permanentmagnets form magnetic fields. In the stator 24, a plurality of windings25 of three phases (U phase, V phase, and W phase) are arranged in acircular ring shape. The windings 25 are classified into windings of afirst system (hereinafter referred to as “first windings”) 26, andwindings of a second system (hereinafter referred to as “secondwindings”) 27. Each set of the windings 26 and 27 is formed by starconnection. In each set of the windings 26 and 27, the windings of therespective phases are alternately arranged along the circumference ofthe stator 24 for each system, collectively arranged side by side alongthe circumference of the stator 24, or arranged at the same teeth whilebeing stacked in a radial direction of the stator 24.

A steering electronic control unit (ECU) 30 is connected to the actuator3. The steering ECU 30 serves as the steering control apparatusconfigured to control driving of the motor 20. The steering ECU 30controls the driving of the motor 20 by controlling supply of a currentthat is a controlled variable of the motor 20 based on detection resultsfrom various sensors provided in the vehicle A. Examples of varioussensors include torque sensors 40 a and 40 b, rotation angle sensors 41a and 41 b, and a vehicle speed sensor 42. The torque sensors 40 a and40 b are provided on the column shaft 11 a. The rotation angle sensors41 a and 41 b are provided on the motor 20. The torque sensors 40 a and40 b detect steering torques Tr1 and Tr2, respectively, that aregenerated with changes on the steering shaft 11 through a user'ssteering operation. The rotation angle sensors 41 a and 41 b detectrotation angles θm1 and θm2, respectively, of the rotary shaft 21 of themotor 20. The vehicle speed sensor 42 detects a vehicle speed V that isa traveling speed of the vehicle A.

The steering ECU 30 is communicably connected to an on-board ADASelectronic control unit (ECU) 43 via a main communication line C1 and asubsidiary communication line C2 of, for example, a controller areanetwork (CAN; registered trademark), which constitute an on-boardnetwork. The steering ECU 30 is communicably connected to the varioussensors 40 a, 40 b, 41 a, 41 b, and 42 via communication lines forsensors (not illustrated), which constitute the on-board network.

The ADAS ECU 43 instructs the steering ECU 30 to perform drivingassistance control for achieving a travel of the vehicle A along atarget course that is set, for example, so that the vehicle A travelswhile keeping its traveling lane or avoiding collisions. The ADAS ECU 43computes the target course to be used for the driving assistance controlbased on image data obtained by photographing with an on-board camera Cmand a detection result from an on-board radar Ra. The target coursecomputed by the ADAS ECU 43 is information indicating the course of thevehicle A, which is a direction of the vehicle A relative to a road.

Next, the electrical configuration of the electric power steering system1 is described.

As illustrated in FIG. 2, the ADAS ECU 43 outputs a main command valueADAS1* to the main communication line C1 in each predetermined period.The main command value ADAS1* is information indicating the targetcourse computed based on the image data obtained by photographing withthe on-board camera Cm and the detection result from the on-board radarRa. The ADAS ECU 43 also outputs a subsidiary command value ADAS2* tothe subsidiary communication line C2 in each predetermined period. Thesubsidiary command value ADAS2* is the same information as the maincommand value ADAS1*, and is structured so as to be usable in place ofthe main command value ADAS1*. Each of the command values ADAS1* andADAS2* is information containing a direction component. In thisembodiment, each of the command values ADAS1* and ADAS2* is structuredso that a rightward direction with respect to the traveling direction ofthe vehicle A is indicated by a positive value (+) and a leftwarddirection with respect to the traveling direction of the vehicle A isindicated by a negative value (−).

The main command value ADAS1* output from the ADAS ECU 43 is input tothe steering ECU 30 via a main transceiver t1 connected to a busterminal of the main communication line C1. The subsidiary command valueADAS2* output from the ADAS ECU 43 is input to the steering ECU 30 via asubsidiary transceiver t2 connected to a bus terminal of the subsidiarycommunication line C2.

The steering ECU 30 includes a main control system 31 and a subsidiarycontrol system 32. The main control system 31 supplies a current to thefirst windings 26 of the motor 20. The subsidiary control system 32supplies a current to the second windings 27 of the motor 20. The maincontrol system 31 includes a main microcomputer 310, a first drivecircuit 311, and a first current detecting circuit 312. The mainmicrocomputer 310 generates a pulse width modulation (PWM) signal P1.The first drive circuit 311 supplies a current to the motor 20 based onthe PWM signal P1. The first current detecting circuit 312 detects acurrent value I1 of each phase, which is generated in a power supplypath between the first drive circuit 311 and the first windings 26. Thesubsidiary control system 32 includes a subsidiary microcomputer 320, asecond drive circuit 321, and a second current detecting circuit 322.The subsidiary microcomputer 320 generates a PWM signal P2. The seconddrive circuit 321 supplies a current to the motor 20 based on the PWMsignal P2. The second current detecting circuit 322 detects a currentvalue I2 of each phase, which is generated in a power supply pathbetween the second drive circuit 321 and the second windings 27. In thisembodiment, the main microcomputer 310 is an example of a first controlprocessing circuit, and the subsidiary microcomputer 320 is an exampleof a second control processing circuit.

The main microcomputer 310 acquires detection results from the torquesensor 40 a, the rotation angle sensor 41 a, and the vehicle speedsensor 42, and also acquires a command value from the ADAS ECU 43. Then,the main microcomputer 310 generates the PWM signal P1 based on thedetection results and the command value, and outputs the PWM signal P1to the first drive circuit 311. Similarly, the subsidiary microcomputer320 acquires detection results from the torque sensor 40 b, the rotationangle sensor 41 b, and the vehicle speed sensor 42, and also acquires acommand value from the ADAS ECU 43. Then, the subsidiary microcomputer320 generates the PWM signal P2 based on the detection results and thecommand value, and outputs the PWM signal P2 to the second drive circuit321.

Specifically, the main microcomputer 310 is connected to the maincommunication line C1 via the main transceiver t1, and to the subsidiarycommunication line C2 via the subsidiary transceiver t2. That is, themain microcomputer 310 is configured to acquire the main command valueADAS1* via the main transceiver t1, and to acquire the subsidiarycommand value ADAS2* via the subsidiary transceiver t2. Similarly, thesubsidiary microcomputer 320 is connected to the main communication lineC1 via the main transceiver t1, and to the subsidiary communication lineC2 via the subsidiary transceiver t2. That is, the subsidiarymicrocomputer 320 is configured to acquire the main command value ADAS1*via the main transceiver t1, and to acquire the subsidiary command valueADAS2* via the subsidiary transceiver t2. The main microcomputer 310 andthe subsidiary microcomputer 320 are configured to communicate necessaryinformation with each other. The microcomputers 310 and 320 shareinformation on, for example, abnormalities of the respectivemicrocomputers and abnormalities of the sensors connected to therespective microcomputers.

Next, functions of the microcomputers 310 and 320 of the steering ECU 30are described in detail. As illustrated in FIG. 3, the mainmicrocomputer 310 includes an assist controlled-variable setting circuit410, an input value processing circuit 411, an ADAS controlled-variablesetting circuit 412, a controlled-variable processing circuit 413, and aPWM output circuit 414. The subsidiary microcomputer 320 includes anassist controlled-variable setting circuit 420 having the same functionas that of the assist controlled-variable setting circuit 410, an inputvalue processing circuit 421 having the same function as that of theinput value processing circuit 411, an ADAS controlled-variable settingcircuit 422 having the same function as that of the ADAScontrolled-variable setting circuit 412, a controlled-variableprocessing circuit 423 having the same function as that of thecontrolled-variable processing circuit 413, and a PWM output circuit 424having the same function as that of the PWM output circuit 414. Themicrocomputers 310 and 320 have the same functions as described above,and therefore the functions of the main microcomputer 310 are mainlydescribed below for convenience.

The steering torque Tr1 and the vehicle speed V are input to the assistcontrolled-variable setting circuit 410. The assist controlled-variablesetting circuit 410 sets and outputs an assist controlled variable TA*based on the steering torque Tr1 and the vehicle speed V. The assistcontrolled variable TA* is a target value of the current to be generatedin the motor 20. The assist controlled variable TA* is a target value ofan assist force to be applied in order to assist the user's steeringoperation, and is a controlled variable for executing electric powersteering control.

The vehicle speed V, the main command value ADAS1*, and the subsidiarycommand value ADAS2* are input to the input value processing circuit411. The input value processing circuit 411 outputs a driving assistancecommand value ADAS* to be used for the driving assistance control basedon the command value ADAS1* or ADAS2*.

The driving assistance command value ADAS* output from the input valueprocessing circuit 411 is input to the ADAS controlled-variable settingcircuit 412. The ADAS controlled-variable setting circuit 412 sets andoutputs an ADAS controlled variable TB* based on the driving assistancecommand value ADAS*. The ADAS controlled variable TB* is a target valueof the current to be generated in the motor 20. The ADAS controlledvariable TB* is a target value of an assist force for assisting theuser's steering operation so that the vehicle A follows the targetcourse computed by the ADAS ECU 43, and is a controlled variable forexecuting the driving assistance control.

The assist controlled variable TA* output from the assistcontrolled-variable setting circuit 410 and the ADAS controlled variableTB* output from the ADAS controlled-variable setting circuit 412 areadded through an addition processing circuit 415. A value obtained bythe addition is input to the controlled-variable processing circuit 413as an assist torque command value T* that is a final target value of thecurrent. The controlled-variable processing circuit 413 generates thePWM signal P1 based on the assist torque command value T*, the rotationangle θm1 acquired from the rotation angle sensor 41 a, and the currentvalue I1 acquired from the first current detecting circuit 312, andoutputs the PWM signal P1 to the first drive circuit 311.

In this embodiment, the microcomputers 310 and 320 are basicallyconfigured such that the main microcomputer 310 functions as a masterprocessing circuit and the subsidiary microcomputer 320 functions as aslave processing circuit.

As indicated by a continuous line in FIG. 3, the assist torque commandvalue T* computed by the main microcomputer 310 and the rotation angleθm1 input to the main microcomputer 310 are output to thecontrolled-variable processing circuit 423 of the subsidiarymicrocomputer 320 through the controlled-variable processing circuit413. The controlled-variable processing circuit 423 of the subsidiarymicrocomputer 320 generates the PWM signal P2 based on the assist torquecommand value T* and the rotation angle θm1 acquired from the mainmicrocomputer 310 and the current value I2 acquired from the secondcurrent detecting circuit 322, and outputs the PWM signal P2 to thesecond drive circuit 321.

As indicated by a dashed line in FIG. 3, the assist controlled-variablesetting circuit 420 of the subsidiary microcomputer 320 sets the assistcontrolled variable TA* similarly to the main microcomputer 310. Theinput value processing circuit 421 of the subsidiary microcomputer 320sets the driving assistance command value ADAS*. The ADAScontrolled-variable setting circuit 422 of the subsidiary microcomputer320 sets the ADAS controlled variable TB*. The assist torque commandvalue T* is input to the controlled-variable processing circuit 423 ofthe subsidiary microcomputer 320.

When a microcomputer abnormality or a sensor abnormality occurs in themain microcomputer 310, the subsidiary microcomputer 320 functions asthe master processing circuit in place of the main microcomputer 310,and the main microcomputer 310 functions as the slave processingcircuit.

In this case, as indicated by dashed lines in FIG. 3, thecontrolled-variable processing circuit 423 of the subsidiarymicrocomputer 320 generates the PWM signal P2 based on the assist torquecommand value T* that is a value obtained by addition through anaddition processing circuit 425, the rotation angle θm2 acquired fromthe rotation angle sensor 41 b, and the current value I2 acquired fromthe second current detecting circuit 322, and outputs the PWM signal P2to the second drive circuit 321.

The assist torque command value T* computed by the subsidiarymicrocomputer 320 and the rotation angle θm2 input to the subsidiarymicrocomputer 320 are output to the controlled-variable processingcircuit 413 of the main microcomputer 310 through thecontrolled-variable processing circuit 423. The controlled-variableprocessing circuit 413 of the main microcomputer 310 generates the PWMsignal P1 based on the assist torque command value T* and the rotationangle θm2 acquired from the subsidiary microcomputer 320 and the currentvalue I1 acquired from the first current detecting circuit 312, andoutputs the PWM signal P1 to the first drive circuit 311. When amicrocomputer abnormality or a sensor abnormality occurs in both of themicrocomputers 310 and 320, the microcomputers 310 and 320 disable thecontrol of the driving of the motor 20.

Next, the functions of the input value processing circuits 411 and 421of the microcomputers 310 and 320 are described in more detail. Theinput value processing circuits 411 and 421 of the microcomputers 310and 320 have the same functions, and therefore the functions of theinput value processing circuit 411 are mainly described below forconvenience.

As illustrated in FIG. 4, the input value processing circuit 411includes an abnormality detecting circuit 510 including a selectionswitching circuit 511, and an input value limiting circuit 512 includinga change amount guarding circuit 513 and an output value switchingcircuit 514.

In the abnormality detecting circuit 510, the main command value ADAS1*and the subsidiary command value ADAS2* are input to the selectionswitching circuit 511. The selection switching circuit 511 is configuredsuch that its selection state is controllable to switch between thecommand values ADAS1* and ADAS2* to be output as a selected commandvalue ADAS(N)*. The selected command value ADAS(N)* is a selected inputvalue to be input to the input value limiting circuit 512 for use in thecontrol of the driving of the motor 20.

The abnormality detecting circuit 510 detects a communication state ofthe main communication line C1 based on the main command value ADAS1*,and also detects a communication state of the subsidiary communicationline C2 based on the subsidiary command value ADAS2*. The abnormalitydetecting circuit 510 controls the selection state of the selectionswitching circuit 511 based on the communication states of thecommunication lines C1 and C2.

Specifically, the abnormality detecting circuit 510 detects a normalstate, a semi-normal state, or an abnormal state of the maincommunication line C1. In the normal state, the main command valueADAS1* is communicable. In the semi-normal state, an abnormality occursbut the main command value ADAS1* is communicable. In the abnormalstate, an abnormality occurs and the main command value ADAS1* is notcommunicable. Those communication states are detected based on a countof failures in the reception of the main command value ADAS1*. Thefailure in the reception of the main command value ADAS1* corresponds toa case where the main command value ADAS1* cannot be received at anappropriate timing, or a case where the main command value ADAS1* can bereceived at an appropriate timing but a checksum indicates anabnormality in its value. The failure in the reception of the maincommand value ADAS1* also corresponds to a case where its absolute valueexceeds an upper limit value in design, or a case where the main commandvalue ADAS1* contains information for notifying the user of anabnormality of the ADAS ECU 43 itself. The same applies to thecommunication state of the subsidiary communication line C2.

For example, as illustrated in FIG. 5, the abnormality detecting circuit510 measures a reception failure count Er that is a count of successivefailures in the reception of the main command value ADAS1*. When thereception failure count Er is smaller than a threshold count Eth(Er<Eth), the abnormality detecting circuit 510 detects that the maincommunication line C1 is in the normal state. For example, the thresholdcount Eth is two or any other number larger than one. The normal statecorresponds to an error active state of the CAN.

When the reception failure count Er is equal to or larger than thethreshold count Eth and this state continues for a period of timeshorter than a threshold time Tth (Er≥Eth), the abnormality detectingcircuit 510 detects that the main communication line C1 is in thesemi-normal state. For example, the threshold time Tth is one second ora few seconds. The semi-normal state is detected during the normal stateto indicate that the main communication line C1 is not in the abnormalstate but an abnormality is detected and therefore the communicationstate has instability. The semi-normal state corresponds to an errorpassive state of the CAN. When the reception failure count Er is smallerthan the threshold count Eth in the semi-normal state, the maincommunication line C1 recovers to the normal state.

When the reception failure count Er is equal to or larger than thethreshold count Eth and this state continues for a period of time equalto or longer than the threshold time Tth (Er≥Eth: Continue), theabnormality detecting circuit 510 detects that the main communicationline C1 is in the abnormal state. The abnormal state is detected duringthe semi-normal state to indicate confirmation of the detection of anabnormality. The abnormal state corresponds to a bus-off state of theCAN. In the abnormal state, the main communication line C1 cannotrecover to the semi-normal state. When a special condition such as areset of communication is satisfied, the main communication line C1recovers to the normal state.

When the main communication line C1 is in the normal state, theabnormality detecting circuit 510 controls the selection state of theselection switching circuit 511 so that the main command value ADAS1* isoutput. That is, when the main communication line C1 is in the normalstate, the abnormality detecting circuit 510 is configured to set theuse of the main command value ADAS1* irrespective of the communicationstate of the subsidiary communication line C2. In this embodiment, themain communication line C1 is a communication line that is basically setfor use in the driving assistance control. The main command value ADAS1*communicated via the main communication line C1 is main controlinformation that is basically set for use in the driving assistancecontrol.

When the main communication line C1 is in the semi-normal state or theabnormal state, that is, when the main communication line C1 is not inthe normal state, the abnormality detecting circuit 510 controls theselection state of the selection switching circuit 511 so that thesubsidiary command value ADAS2* is output under a condition that thesubsidiary communication line C2 is in the normal state. Thus, when themain communication line C1 is not in the normal state, the abnormalitydetecting circuit 510 is configured to set the use of the subsidiarycommand value ADAS2* under the condition that the subsidiarycommunication line C2 is in the normal state. In this embodiment, thesubsidiary communication line C2 is a communication line that is set foruse in the driving assistance control as a substitute when the maincommunication line C1 is not in the normal state and the communicationstate has instability. The subsidiary command value ADAS2* communicatedvia the subsidiary communication line C2 is subsidiary controlinformation that is set for use in the driving assistance control as asubstitute when the main communication line C1 is not in the normalstate and the communication state has instability. When neither thecommunication line C1 nor C2 is in the normal state, the abnormalitydetecting circuit 510 performs control so as to maintain the selectionstate of the selection switching circuit 511 at that time.

In the input value limiting circuit 512, the selected command valueADAS(N)*, the vehicle speed V, and a switched command value ADAS(N−1)*are input to the change amount guarding circuit 513. The switchedcommand value ADAS(N−1)* is the driving assistance command value ADAS*that is used for the driving assistance control last time. The changeamount guarding circuit 513 is configured to generate a limiting commandvalue Rg* based on the selected command value ADAS(N)*, the vehiclespeed V, and the switched command value ADAS(N−1)*, and to output thelimiting command value Rg* to the output value switching circuit 514.

Specifically, the change amount guarding circuit 513 outputs thelimiting command value Rg* when the selected command value ADAS(N)*deviates from an allowable range defined in accordance with the vehiclespeed V. In this embodiment, the allowable range is based on theswitched command value ADAS(N−1)* and is set by a limiting threshold Gthdefined in accordance with the vehicle speed V. The allowable range is arange that is equal to or smaller than the absolute value of a valueobtained by adding the switched command value ADAS(N−1)* and thelimiting threshold Gth. The limiting command value Rg* is a valueobtained by adding the switched command value ADAS(N−1)* and thelimiting threshold Gth, and is a maximum or minimum value of theallowable range.

As illustrated in FIG. 6, the change amount guarding circuit 513 has amap that defines a relationship between the vehicle speed V and theabsolute value of the limiting threshold Gth. In this map, the absolutevalue of the limiting threshold Gth is set smaller as the vehicle speedV is larger. Each of the selected command value ADAS(N)* and theswitched command value ADAS(N−1)* is information containing a directioncomponent similarly to the command values ADAS1* and ADAS2*. Thelimiting threshold Gth is information containing a direction component,and is basically set as a positive value (+) or a negative value (−) sothat the direction component is identical to the direction component ofthe switched command value ADAS(N−1)*.

For example, when the selection state of the selection switching circuit511 is switched from the selection state in which the main command valueADAS1* is output as the selected command value ADAS(N)* to the selectionstate in which the subsidiary command value ADAS2* is output as theselected command value ADAS(N)*, the selected command value ADAS(N)* maydeviate unexpectedly so as to change abruptly before and after theswitching. The limiting threshold Gth is a value set within a range thatis experimentally determined under the assumption that the selectedcommand value ADAS(N)* does not change abruptly before and after theswitching of the selection state of the selection switching circuit 511.

In the input value limiting circuit 512, the selected command valueADAS(N)* and the limiting command value Rg* are input to the outputvalue switching circuit 514. The output value switching circuit 514 isconfigured such that its selection state is controllable to switchbetween the selected command value ADAS(N)* and the limiting commandvalue Rg* to be output as the driving assistance command value ADAS*.

When the input selected command value ADAS(N)* is a value obtained afterthe selection state of the selection switching circuit 511 is switched,the input value limiting circuit 512 detects whether the selectedcommand value ADAS(N)* deviates from the allowable range that is set bythe change amount guarding circuit 513. The input value limiting circuit512 controls the selection state of the output value switching circuit514 based on the input selected command value ADAS(N)*.

Specifically, when the input selected command value ADAS(N)* is not thevalue obtained after the selection state of the selection switchingcircuit 511 is switched, the input value limiting circuit 512 controlsthe selection state of the output value switching circuit 514 so thatthe selected command value ADAS(N)* is output as the driving assistancecommand value ADAS*. The same applies to a case where the input selectedcommand value ADAS(N)* is the value obtained after the selection stateof the selection switching circuit 511 is switched but does not deviatefrom the allowable range.

When the input selected command value ADAS(N)* is the value obtainedafter the selection state of the selection switching circuit 511 isswitched and deviates from the allowable range, the input value limitingcircuit 512 controls the selection state of the output value switchingcircuit 514 so that the limiting command value Rg* is output as thedriving assistance command value ADAS*.

Next, subsidiary information effectiveness determining processing (FIG.8), main information effectiveness determining processing (FIG. 9), andinformation selecting processing (FIG. 10) to be executed by theabnormality detecting circuit 510 and input value limiting processing(FIG. 11) to be executed by the input value limiting circuit 512 aredescribed in detail regarding input value processing to be executed bythe input value processing circuit 411 in order to output the drivingassistance command value ADAS* (FIG. 7). Also in this case, the inputvalue processing circuits 411 and 421 of the microcomputers 310 and 320have the same functions, and therefore the functions of the input valueprocessing circuit 411 are mainly described below for convenience.

When the driving assistance control is enabled, the input valueprocessing circuit 411 executes the input value processing in eachcontrol period of the main microcomputer 310. As illustrated in FIG. 7,the abnormality detecting circuit 510 of the input value processingcircuit 411 first executes the subsidiary information effectivenessdetermining processing (Step S10), the main information effectivenessdetermining processing (Step S20), and the information selectingprocessing (Step S30) in this order. Then, the input value limitingcircuit 512 of the input value processing circuit 411 executes the inputvalue limiting processing (Step S40), and then the abnormality detectingcircuit 510 executes the subsidiary information effectivenessdetermining processing (Step S10) again.

Specifically, as illustrated in FIG. 8, in the subsidiary informationeffectiveness determining processing (Step S10), the abnormalitydetecting circuit 510 determines whether the subsidiary communicationline C2 is in the normal state (Step S11). When the subsidiarycommunication line C2 is in the normal state (Step S11: YES), theabnormality detecting circuit 510 sets “1” to a subsidiary informationeffectiveness flag (FLG) (Step S12). Then, the abnormality detectingcircuit 510 terminates the subsidiary information effectivenessdetermining processing, and returns to the input value processing. Thesubsidiary information effectiveness FLG is information indicating thecommunication state of the subsidiary communication line C2, and is setin a predetermined storage area of the main microcomputer 310.

When the subsidiary communication line C2 is not in the normal state(Step S11: NO), the abnormality detecting circuit 510 sets “0” to thesubsidiary information effectiveness FLG (Step S13). Then, theabnormality detecting circuit 510 terminates the subsidiary informationeffectiveness determining processing, and returns to the input valueprocessing.

As illustrated in FIG. 9, in the main information effectivenessdetermining processing (Step S20), the abnormality detecting circuit 510determines whether the main communication line C1 is in the normal state(Step S21). When the main communication line C1 is in the normal state(Step S21: YES), the abnormality detecting circuit 510 sets “1” to amain information effectiveness FLG (Step S22). Then, the abnormalitydetecting circuit 510 terminates the main information effectivenessdetermining processing, and returns to the input value processing. Themain information effectiveness FLG is information indicating thecommunication state of the main communication line C1, and is set in apredetermined storage area of the main microcomputer 310.

When the main communication line C1 is not in the normal state (StepS21: NO), the abnormality detecting circuit 510 sets “0” to the maininformation effectiveness FLG (Step S23). Then, the abnormalitydetecting circuit 510 terminates the main information effectivenessdetermining processing, and returns to the input value processing.

As illustrated in FIG. 10, in the information selecting processing (StepS30), the abnormality detecting circuit 510 determines whether “1” isset to the main information effectiveness FLG (Step S31). When “1” isset to the main information effectiveness FLG (Step S31: YES), theabnormality detecting circuit 510 determines that the main communicationline C1 is in the normal state, and sets the use of the main commandvalue ADAS1* communicated via the main communication line C1 (Step S32).In Step S32, the abnormality detecting circuit 510 controls theselection state of the selection switching circuit 511 so that the maincommand value ADAS1* is output. Then, the abnormality detecting circuit510 sets “1” to a selection FLG (Step S33). Then, the abnormalitydetecting circuit 510 terminates the information selecting processing,and returns to the input value processing. The selection FLG isinformation indicating the selection state of the selection switchingcircuit 511, and is set in a predetermined storage area of the mainmicrocomputer 310. As the selection FLG, a selection FLG(N) and aselection FLG(N−1) are stored. The selection FLG(N) indicates a latestcontent. The selection FLG(N−1) indicates a previous content.

When “1” is not set to the main information effectiveness FLG (Step S31:NO), the abnormality detecting circuit 510 determines whether “1” is setto the subsidiary information effectiveness FLG. That is, theabnormality detecting circuit 510 determines whether “0” is set to themain information effectiveness FLG and “1” is set to the subsidiaryinformation effectiveness FLG (Step S34). When “0” is set to the maininformation effectiveness FLG and “1” is set to the subsidiaryinformation effectiveness FLG (Step S34: YES), the abnormality detectingcircuit 510 determines that the main communication line C1 is not in thenormal state and the subsidiary communication line C2 is in the normalstate, and sets the use of the subsidiary command value ADAS2*communicated via the subsidiary communication line C2 (Step S35). InStep S35, the abnormality detecting circuit 510 controls the selectionstate of the selection switching circuit 511 so that the subsidiarycommand value ADAS2* is output. Then, the abnormality detecting circuit510 sets “2” to the selection FLG (Step S36). Then, the abnormalitydetecting circuit 510 terminates the information selecting processing,and returns to the input value processing.

When “0” is set to the main information effectiveness FLG and “1” is notset to the subsidiary information effectiveness FLG (Step S34: NO), theabnormality detecting circuit 510 determines that neither thecommunication line C1 nor C2 is in the normal state, and maintains thesetting of the use of the command value ADAS1* or ADAS2* at that time(Step S37). In Step S37, the abnormality detecting circuit 510 maintainsthe selection state of the selection switching circuit 511 at that time,and also maintains the content of the selection FLG. Then, theabnormality detecting circuit 510 terminates the information selectingprocessing, and returns to the input value processing.

As illustrated in FIG. 11, in the input value limiting processing (StepS40), the input value limiting circuit 512 determines whether theselection FLG(N) and the selection FLG(N−1) do not match each other(Step S41). When the selection FLG(N) and the selection FLG(N−1) matcheach other (Step S41: NO), the input value limiting circuit 512determines that the selection state of the selection switching circuit511 is not switched, and proceeds to processing of Step S43.

When the selection FLG(N) and the selection FLG(N−1) do not match eachother (Step S41: YES), the input value limiting circuit 512 determinesthat the selected command value ADAS(N)* to be used after the selectionstate of the selection switching circuit 511 is switched may deviatefrom the allowable range. Then, the input value limiting circuit 512sets “1” to a switching FLG (Step S42), and proceeds to the processingof Step S43. The switching FLG is information indicating whether theselection state of the selection switching circuit 511 is switched andthe selected command value ADAS(N)* to be used after the switching maydeviate from the allowable range. The switching FLG is set in apredetermined storage area of the main microcomputer 310.

The input value limiting circuit 512 that proceeds to Step S43 from StepS41 or Step S42 determines whether “1” is set to the switching FLG. When“1” is not set to the switching FLG (Step S43: NO), the input valuelimiting circuit 512 sets the use of the selected command value ADAS(N)*(Step S44). In Step S44, the input value limiting circuit 512 controlsthe selection state of the output value switching circuit 514 so thatthe selected command value ADAS(N)* is output. Then, the input valuelimiting circuit 512 terminates the input value limiting processing, andreturns to the input value processing.

When “1” is set to the switching FLG (Step S43: YES), the input valuelimiting circuit 512 determines whether the absolute value of theselected command value ADAS(N)* (|ADAS(N)*|) is larger than the absolutevalue of a value obtained by adding the switched command valueADAS(N−1)* and the limiting threshold Gth (|ADAS(N−1)*+Gth|) (Step S45).In Step S45, the change amount guarding circuit 513 of the input valuelimiting circuit 512 determines whether the selected command valueADAS(N)* deviates from the allowable range.

When the selected command value ADAS(N)* deviates from the allowablerange (Step S45: YES), the input value limiting circuit 512 sets the useof the limiting command value Rg* (Step S46). In Step S46, the inputvalue limiting circuit 512 controls the selection state of the outputvalue switching circuit 514 so that the limiting command value Rg* isoutput. Then, the input value limiting circuit 512 terminates the inputvalue limiting processing, and returns to the input value processing.

When the selected command value ADAS(N)* does not deviate from theallowable range (Step S45: NO), the input value limiting circuit 512sets the use of the selected command value ADAS(N)* (Step S47). In StepS47, the input value limiting circuit 512 controls the selection stateof the output value switching circuit 514 so that the selected commandvalue ADAS(N)* is output. Then, the input value limiting circuit 512sets “0” to the switching FLG (Step S48). Then, the input value limitingcircuit 512 terminates the input value limiting processing, and returnsto the input value processing. In Step S48, the input value limitingcircuit 512 sets “0” to the switching FLG because the selection state ofthe selection switching circuit 511 is switched but the selected commandvalue ADAS(N)* does not deviate from the allowable range.

Actions and effects of this embodiment are described below.

(1) In the information selecting processing (FIG. 10), when the maincommunication line C1 is in the normal state, the abnormality detectingcircuit 510 sets the use of the main command value ADAS1*. When the maincommunication line C1 is not in the normal state, the abnormalitydetecting circuit 510 sets the use of the subsidiary command valueADAS2* under the condition that the subsidiary communication line C2 isin the normal state.

Therefore, the use of the main command value ADAS1* communicated via themain communication line C1 having instability in its communication statecan be stopped in a stage in which the main communication line C1 is inthe semi-normal state before the abnormal state is detected. In thiscase, detection of the normal state for the subsidiary communicationline C2 is set as the condition. Therefore, the driving assistancecontrol can be continued by using the subsidiary command value ADAS2* inplace of the main command value ADAS1*. Thus, it is possible to reducethe occurrence of a situation in which the command value communicatedvia the communication line having instability or abnormality in itscommunication state is used for the driving assistance control.Accordingly, it is possible to reduce the occurrence of a situation inwhich the assist force cannot appropriately be applied to the steeringmechanism 2 even in the situation in which the communication state ofthe main communication line C1 has instability.

(2) In the input value processing (FIG. 7), the abnormality detectingcircuit 510 first detects the communication state of the subsidiarycommunication line C2, and then detects the communication state of themain communication line C1. Therefore, when the semi-normal state isdetected for the main communication line C1, it is possible to reducethe occurrence of a situation in which the driving assistance control iscontinued by using the subsidiary command value ADAS2* in place of themain command value ADAS1* even though the subsidiary communication lineC2 is not in the normal state. Accordingly, it is possible to moresuitably reduce the occurrence of the situation in which the assistforce cannot appropriately be applied to the steering mechanism 2 evenin the situation in which the communication state of the maincommunication line C1 has instability.

(3) In the input value processing (FIG. 7), the abnormality detectingcircuit 510 detects the communication states of the main communicationline C1 and the subsidiary communication line C2 in each control period,and based on the detection results, sets the use of any one of thecommand values ADAS1* and ADAS2* in each control period. Therefore, whenthe main communication line C1 recovers to the normal state after thedriving assistance control is continued by using the subsidiary commandvalue ADAS2* because the main communication line C1 changes to a stateother than the normal state, a recovery can be made such that thedriving assistance control is executed by using the main command valueADAS1*.

Specifically, when the main communication line C1 recovers to the normalstate after the main communication line C1 changes to a state other thanthe normal state, the abnormality detecting circuit 510 controls theselection state of the selection switching circuit 511 so that the maincommand value ADAS1* is output. That is, when the main communicationline C1 is in the normal state, the abnormality detecting circuit 510 isconfigured to set the use of the main command value ADAS1* irrespectiveof whether the main communication line C1 recovers to the normal stateafter the main communication line C1 changes to a state other than thenormal state.

When neither the communication line C1 nor C2 is in the normal state,the abnormality detecting circuit 510 maintains the selection state ofthe selection switching circuit 511 at that time. When the maincommunication line C1 recovers to the normal state, the abnormalitydetecting circuit 510 controls the selection state of the selectionswitching circuit 511 so that the main command value ADAS1* is output.That is, when the main communication line C1 is in the normal state, theabnormality detecting circuit 510 is configured to set the use of themain command value ADAS1* even if the main communication line C1recovers to the normal state after each of the communication lines C1and C2 changes to a state other than the normal state. Accordingly, thisembodiment is effective in securing a longer period of time for thesituation in which the assist force can appropriately be applied to thesteering mechanism 2 in the driving assistance control.

(4) In JP 2007-153001 A described in “BACKGROUND OF THE INVENTION”, thecontent of the identification information is switched, so that thecurrent communication line of the backup system is switched to acommunication line of an active system. Before and after the switching,however, the value of the information for controlling the driving of themotor may deviate unexpectedly. That is, when the communication line ofthe backup system is switched to a communication line of an activesystem, the value of the information for controlling the driving of themotor may change abruptly before and after the switching. Thus, there isa situation in which the power cannot appropriately be applied to thesteering mechanism.

In view of the above, in the input value limiting processing (FIG. 11),when the selected command value ADAS(N)* deviates from the allowablerange after the selection state of the selection switching circuit 511is switched, the input value limiting circuit 512 sets the use of thelimiting command value Rg*.

Therefore, even when the selection state of the selection switchingcircuit 511 is switched and the selected command value ADAS(N)* to beused after the switching deviates unexpectedly, the deviation issuppressed by setting the use of the limiting command value Rg*. Thus,it is possible to reduce influence of the switching of the selectionstate of the selection switching circuit 511 on the driving assistancecontrol. Accordingly, it is possible to reduce the occurrence of asituation in which the assist force cannot appropriately be applied tothe steering mechanism 2 even after the selection state of the selectionswitching circuit 511 is switched.

(5) The allowable range is based on the switched command valueADAS(N−1)* and is set by the limiting threshold Gth that defines itsrelationship in accordance with the vehicle speed V of the vehicle Aafter the selection state of the selection switching circuit 511 isswitched.

Therefore, when the selection state of the selection switching circuit511 is switched and the selected command value ADAS(N)* to be used afterthe switching deviates unexpectedly, it is possible to use the limitingcommand value Rg* that is optimized in accordance with the travelingstate of the vehicle A at that time, that is, the vehicle speed V of thevehicle A. Accordingly, it is possible to more suitably reduce theoccurrence of the situation in which the assist force cannotappropriately be applied to the steering mechanism 2 even after theselection state of the selection switching circuit 511 is switched.

(6) In the input value limiting processing (FIG. 11), when the selectionstate of the selection switching circuit 511 is switched and then theselected command value ADAS(N)* does not deviate from the allowablerange during a period in which the driving assistance control iscontinued by using the limiting command value Rg*, the input valuelimiting circuit 512 sets the use of the selected command valueADAS(N)*.

Therefore, it is possible to suppress protraction of a situation inwhich the use of the limiting command value Rg* is set even after theselection state of the selection switching circuit 511 is switched.Thus, it is possible to promptly recover to the state in which thedriving assistance control is executed by using the command value ADAS1*or ADAS2* even after the selection state of the selection switchingcircuit 511 is switched.

The embodiment described above may be modified as appropriate andimplemented as in the following modified embodiments. The steering ECU30 may include the main microcomputer 310 alone. In the steering ECU 30,the main communication line C1 may be connected to the mainmicrocomputer 310 alone, and the subsidiary communication line C2 may beconnected to the subsidiary microcomputer 320 alone. In this case, it isonly necessary that the communication states of the communication linesrespectively connected to the microcomputers 310 and 320 be communicablebetween the microcomputers 310 and 320. In this case, the selectionstate of the selection switching circuit 511 and the switching betweenthe microcomputers 310 and 320 for the master processing circuit and theslave processing circuit may be associated with each other. For example,it is only necessary that the microcomputer to which the communicationline to be used for the driving assistance control is connected be setas the master processing circuit and the other microcomputer be set asthe slave processing circuit.

Instead of providing the input value processing circuits 411 and 421 inthe microcomputers 310 and 320, respectively, a computation circuit suchas an application-specific integrated circuit (ASIC) having thefunctions of the input value processing circuits 411 and 421 may beprovided separately from the microcomputers 310 and 320. In this case,individual computation circuits may be provided for the microcomputers310 and 320, or a single common computation circuit may be provided forthe microcomputers 310 and 320.

In the input value processing, the subsidiary information effectivenessdetermining processing, the main information effectiveness determiningprocessing, and the information selecting processing need not beexecuted after the selection state of the selection switching circuit511 is switched so that the subsidiary command value ADAS2* is outputthrough the processing of Step S35 of the information selectingprocessing.

The order of execution of the subsidiary information effectivenessdetermining processing and the main information effectivenessdetermining processing may be set reversely. It is only necessary thatthe information selecting processing be executed after the subsidiaryinformation effectiveness determining processing and the maininformation effectiveness determining processing are executed in thesame control period of the microcomputers 310 and 320.

Once “1” is set to the switching FLG, the switching FLG in this statemay be continued unless a special condition such as a reset ofcommunication is satisfied.

The map that defines the limiting threshold Gth may define arelationship between the limiting threshold Gth and a rotation angle ofthe column shaft 11 a and steering angles of the steered wheels 15, thatis, an actual angle θs, a steering angle velocity ω indicating a changeamount of the actual angle θs, or the current values I1 and I2 of themotor 20 instead of the vehicle speed V. The map may also define arelationship between the limiting threshold Gth and any one of thosefactors or a combination of those factors and the vehicle speed V.

The limiting threshold Gth may be a value that is set in advanceirrespective of the traveling state of the vehicle A, that is, thevehicle speed V. Other methods are applicable as long as the limitingcommand value Rg* is generated as a value that falls within theallowable range used in Step S45 of the input value limiting processing.

Regarding the communication state, it is only necessary to determinethat a reception failure occurs in the main communication line C1 atleast in the case where the main command value ADAS1* cannot be receivedat an appropriate timing, or the case where the main command valueADAS1* can be received at an appropriate timing but the checksumindicates an abnormality in its value. The same applies to thecommunication state of the subsidiary communication line C2.

Regarding the communication state, it may be determined whether afailure in the reception of the main command value ADAS1* occurs in themain communication line C1 by detecting various voltages such as anoperation voltage for the operation of the main microcomputer 310. Thesame applies to the communication state of the subsidiary communicationline C2.

Regarding the communication state, it may be detected that thecommunication state is abnormal when the reception failure count Er isequal to or larger than a second threshold count that is larger than thethreshold count Eth. The main communication line C1 and the subsidiarycommunication line C2 may be changed so as to use a communication schemesuch as CAN with Flexible Data Rate (CAN FD) or FlexRay (registeredtrademark). Three lines may be used by adding a subsidiary communicationline C3 having a function similar to that of the subsidiarycommunication line C2 in addition to the communication lines C1 and C2.Further, four or more lines may be used by adding subsidiarycommunication lines having similar functions.

In the embodiment described above, each of the command values ADAS1* andADAS2* may be implemented by, for example, an angle command valueindicating an angle of the vehicle A relative to a road, or a torquecommand value indicating a torque for turning the steered wheels 15 soas to achieve the angle of the vehicle A relative to the road. When theangle command value is used as each of the command values ADAS1* andADAS2*, the ADAS controlled-variable setting circuits 412 and 422 onlyneed to calculate the rotation angle of the column shaft 11 a and thesteering angles of the steered wheels 15, that is, the actual angle θsby processing the rotation angles θm1 and θm2, and to set the controlledvariables based on the actual angle θs. When the torque command value isused as each of the command values ADAS1* and ADAS2*, the ADAScontrolled-variable setting circuits 412 and 422 only need to set thetorque command values directly as the controlled variables.

The driving assistance control may assist the user's driving by a methoddifferent from the method of following the target course. For example,the driving assistance control may be implemented by automatic brakingassistance control for automatically applying a brake, or by sideslippreventing control (vehicle stability control).

When the assist controlled variable TA* is set, the use of the steeringtorque Tr1 or Tr2 is at least required, and the use of the vehicle speedV is not necessarily required. When the assist controlled variable TA*is set, other factors may be used in addition to the steering torque Tr1or Tr2 and the vehicle speed V. When the ADAS controlled variable TB* isset, the use of the command value ADAS1* or ADAS2* is at least required.The vehicle speed V and other factors may be used in addition to thecommand value ADAS1* or ADAS2*.

The embodiment described above is applied to the electric power steeringsystem 1 of the type in which the power is applied to the column shaft11 a, but may be applied to an electric power steering system of a typein which the power is applied to the rack shaft 12. In this case, thetorque sensors 40 a and 40 b may be provided on, for example, the pinionshaft 11 c.

The embodiment described above is also applicable to, for example, asteer-by-wire type steering system. In this case, it is only necessarythat the actuator 3 drive the rack shaft 12 as a drive unit provided onthe periphery of the rack shaft 12. The modified examples may be appliedin combination. For example, the embodiment applied to the steer-by-wiretype steering system and the configurations of the other modifiedexamples may be applied in combination.

What is claimed is:
 1. A steering control apparatus for use with a motorthat is a source of power for turning a steered wheel of a vehicle suchthat the power is applied to a steering mechanism through the motor, thesteering control apparatus comprising: control processing circuitsconfigured to control driving of the motor so that the power for turningthe steered wheel is applied to the steering mechanism; a plurality ofcommunication lines for communicating control information to the controlprocessing circuits for controlling the driving of the motor, theplurality of communication lines being connected to each of the controlprocessing circuits, the plurality of communication lines including: amain communication line for communicating the control information to thecontrol processing circuits, and a subsidiary communication line forcommunicating the control information to the control processingcircuits, and an abnormality detecting circuit configured to: detect, asa communication state of each of the plurality of communication lines,(i) a normal state in which the control information is communicable,(ii) a semi-normal state detected during the normal state, thesemi-normal state being a state in which an abnormality occurs but thecontrol information is communicable through the main communication line,or (iii) an abnormal state detected during the semi-normal state, theabnormal state being a state in which an abnormality occurs and thecontrol information is not communicable through the main communicationline; when the normal state is detected for the main communication line,set use of communicating the control information via the maincommunication line; and when the semi-normal state or the abnormal stateis detected for the main communication line, set use of communicatingthe control information via the subsidiary communication line under acondition that the normal state is detected for the subsidiarycommunication line.
 2. The steering control apparatus according to claim1, wherein the abnormality detecting circuit is configured to firstdetect the communication state of the subsidiary communication line, andthen detect the communication state of the main communication line. 3.The steering control apparatus according to claim 1, wherein theabnormality detecting circuit is configured to: detect the communicationstates of the main communication line and the subsidiary communicationline in control periods defined in advance, and set use of any one ofthe main control information and the subsidiary control information inthe each control period.